Modified Vaccinia Virus Ankara (MVA) is a member of the Orthopoxvirus family and has been generated by about 570 serial passages on chicken embryo fibroblasts of the Ankara strain of Vaccinia virus (CVA) (for review see Mayr, A., et al. [1975], Infection 3, 6-14). As a consequence of these passages the resulting MVA virus contains 31 kilobases less genomic information compared to CVA and is highly host cell restricted (Meyer, H. et al., J. Gen. Virol. 72, 1031-1038 [1991]). MVA is characterized by its extreme attenuation, namely by a diminished virulence or infectiosity but still an excellent immunogenicity. When tested in a variety of animal models, MVA was proven to be avirulent even in immuno-suppressed individuals. More importantly, the excellent properties of the MVA strain have been demonstrated in extensive clinical trials (Mayr et al., Zbl. Bakt. Hyg. I, Abt. Org. B 167, 375-390 [1987]). During these studies in over 120,000 humans, including high risk patients, no side effects were seen (Stickl et al., Dtsch. med. Wschr. 99, 2386-2392 [1974]).
It has been further found that MVA is blocked in the late stage of the virus replication cycle in mammalian cells (Sutter, G. and Moss, B. [1992] Proc. Natl. Acad. Sci. USA 89, 10847-10851). Accordingly, MVA fully replicates its DNA, synthesizes early, intermediate and late gene products, but is not capable to assemble mature infectious virions, which could be released from an infected cell. For this reason, namely to be replication restricted, MVA was proposed to serve as a gene expression vector.
More recently, MVA was used to generate recombinant vaccines, expressing antigenic sequences inserted either at the site of the tymidine-kinase (tk) gene (U.S. Pat. No. 5,185,146) or at the site of a naturally occurring deletion within the MVA genome (PCT/EP96/02926).
Although the tk insertion locus is widely used for the generation of recombinant poxviruses, particularly for the generation of recombinant Vaccinia viruses (Mackett, et al. [1982] P.N.A.S. USA 79, 7415-7419) this technology was not applicable for MVA. It was shown by Scheiflinger et al., that MVA is much more sensitive to modifications of the genome compared to other poxviruses, which can be used for the generation of recombinant poxviruses. Scheiflinger et al. showed in particular that one of the most commonly used site for the integration of heterologous DNA into poxviral genomes, namely the thymdine kinase (tk) gene locus, cannot be used to generate recombinant MVA. Any resulting tk(−) recombinant MVA proved to be highly unstable and upon purification immediately deleted the inserted DNA together with parts of the genomic DNA of MVA (Scheiflinger et al. [1996], Arch Virol 141: pp 663-669).
Instability and, thus, high probability of genomic recombination is a known problem within pox virology. Actually, MVA was established during long-term passages exploiting the fact that the viral genome of CVA is unstable when propagated in vitro in tissue cultured cells. Several thousands of nucleotides (31 kb) had been deleted from the MVA genome, which therefore is characterized by 6 major and numerous small deletions in comparison to the original CVA genome.
The genomic organization of the MVA genome has been described recently (Antoine et al. [1998], Virology 244, 365-396). The 178 kb genome of MVA is densely packed and comprises 193 individual open reading frames (ORFs), which code for proteins of at least 63 amino acids in length. In comparison with the highly infectious Variola virus and also the prototype of Vaccinia virus, namely the strain Copenhagen, the majority of ORFs of MVA are fragmented or truncated (Antoine et al. [1998], Virology 244, 365-396). However, with very few exceptions all ORFs, including the fragmented and truncated ORFs, get transcribed and translated into proteins. In the following, the nomenclature of Antoine et al. is used and —where appropriate—the nomenclature based on Hind III restriction enzyme digest is also indicated.
So far, only the insertion of exogenous DNA into the naturally occurring deletion sites of the MVA genome led to stable recombinant MVAs (PCT/EP96/02926). Unfortunately, there is only a restricted number of naturally occurring deletion sites in the MVA genome. Additionally it was shown that other insertion sites, such as, e.g., the tk gene locus, are hardly useful for the generation of recombinant MVA (Scheiflinger et al. [1996], Arch Virol 141: pp 663-669).